Deficiency in microsomal glucose-6-phosphatase (G6Pase) activity causes glycogen storage disease type 1 (GSD-1), which has been divided into four subgroups, 1a, 1b, 1c and 1d, corresponding to defects in components of the G6Pase system: the G6Pase catalytic unit (1a), a putative G6P translocase (1b), a putative phosphate/pyrophosphate translocase (1c), and a putative glucose translocase (1d). Using G6Pase knockout mice which mimic the pathophysiology of human GSD-1a, we have demonstrated that G6P transport and hydrolysis are performed by separate proteins. We now show that G6P hydrolysis, G6P transport, and pyrophosphate transport have markedly different expression patterns, both from each other and from those in the liver and in the kidney. Moreover, hepatic microsomal G6P transport activity between neonatal and adult mice differ markedly, raising the possibility that differential screening may allow the cloning and characterization of GSD-1b gene. The G6Pase gene is expressed in a tissue-specific manner in the liver and kidney. We demonstrate that DNA elements essential for optimal and liver-specific expression of the G6Pase gene are contained within nucleotides -234 to +3. The G6Pase promoter contains three activation elements (AE-I, II, and III) and multiple binding sites for transcription factors, including HNF1, HNF3, CREB, and C/EBP. We show that HNF1alpha binds to its cognate site within AE-I and HNF3gamma binds to its cognate sites within AE-II and AE-III; both transactivate G6Pase gene expression. AE-II, which forms multiple protein-DNA complexes, including HNF3gamma, CREB, C/EBPalpha, and C/EBPbeta, mediates transcription activation of the G6Pase gene by cAMP. Methionine adenosyltransferase (MAT) I/III deficiency, characterized by isolated persistent hypermethioninemia, is caused by mutations in the MAT1A gene encoding MATalpha1, the subunit of major hepatic enzymes, MAT I [(alpha1)4] and III [(alpha1)2]. We have characterized ten MAT1A mutations in MAT I/III deficient individuals and shown that the associated hypermethioninemic phenotype was inherited as an autosomal recessive trait. However, a single allelic R264H mutation is shown to associate with an autosomal dominant pattern of inheritance, caused by the formation of R264/R264H MATalpha1 heterodimers that are enzymatically inactive.